QUANTITATIVE MICROSCOPY 491 interaction of detergents with fat films or deposits, the formation of myelin figures, the structure of creams, foams, lathers, and protective colloids, to name a few examples. Figure 5 show• the interaction between a hair- washing agent and a fat film. The examination of oil, water or water/oil emulsions involves study of pure phase objects. Phase contrast permits a definite distinction between lipid and aqueous phases and their structural distribution (Fig. 6). A quantitative determination of the respective partial volumes of the two phases may employ the same principles as those used for the determination of partial volumes in mineralogy and ore microscopy (8, 9). It can be shown that, with certain assumptions, the relative areas are directly pro- 2OO I d 700 600 5 O0 400 m•zz Figure &--Absorption spectra of human hair (coefficient of absorption = 1/d In Io/I. bk = black dbd = dark blond dr = titian red bd == blond bl --- bleached.

492 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS portional to the partial volumes. Such an analysis is carried out either with an integrating stage, a point-counting device, or a photomicrograph. Phase contrast techniques can also be used in reflected light. It is then necessary that the specimen surface be relatively smooth and not give too diffuse a reflection. The sensitivity for detection of relief, or optic path difference in general, is higher in reflected light than in transmitted light. The sensitivity for detection of surface relief is approximately four times higher in incident than in transmitted light phase contrast. In incident or reflected light the path difference enters twice. This causes a gain of a factor of 2 in sensitivity. In transmitted light, one would have to examine a replica of the same relief. With a replica refractive index of 1.5, the ef- fective path difference against air would only be 1.5 - 1.0 = 0.5 of the 700 660 560 460 rn,a Figure 4.--Reflection spectra of human hair. bl =- bleached hair dr = titian red lr = light red bd = blond dbd = dark blond bk = black wp = white paper surface.